This invention relates to a method for making an optical compensator for improving viewing-angle characteristics of liquid crystal displays.
Current rapid expansion in the use of liquid crystal displays (LCDs), in various areas of information display, is largely due to improvements of display qualities. Contrast, color reproduction, and stable gray scale intensities are important display qualities for electronic displays, which employ liquid crystal technology. The primary factor limiting the contrast of a liquid crystal display is the propensity for light to xe2x80x9cleakxe2x80x9d through liquid crystal elements or cell, which are in the dark or xe2x80x9cblackxe2x80x9d pixel state. Furthermore, the leakage and hence contrast of a liquid crystal display are also dependent on the angle from which the display screen is viewed. Typically the optimum contrast is observed only within a narrow viewing angle centered about normal incidence to the display and falls off rapidly as the viewing angle is increased. In color displays, the leakage problem not only degrades the contrast, but also causes color or hue shifts with an associated degradation of color reproduction. In addition to black-state light leakage, the narrow viewing angle problem in typical twisted nematic liquid crystal displays is exacerbated by a shift in the brightness-voltage curve as a function of viewing angle because of the optical anisotropy of the liquid crystal material.
Thus, one of the major considerations for evaluating the quality of such displays is the viewing-angle characteristics, which describes a change in contrast ratio from different viewing angles. It is desirable to be able to see the same image from a wide variation in viewing angles and this ability has been a shortcoming with liquid crystal display devices. One way to improve the viewing-angle characteristics is to insert an optical compensator (also referred to as a compensation film, retardation film, or retarder) situated between the polarizer and liquid crystal cell. An optical compensator can widen the viewing-angle characteristics of liquid crystal displays, and in particular of Twisted Nematic (TN), Super Twisted Nematic (STN), Optically Compensated Bend (OCB), In Plane Switching (IPS), or Vertically Aligned (VA) liquid crystal displays. These various liquid crystal display technologies have been reviewed in U.S. Pat. No. 5,619,352 (Koch et al.), U.S. Pat. No. 5,410,422 (Bos), and U.S. Pat. No. 4,701,028 (Clerc et al.).
Optical compensators are disclosed in U.S. Pat. No. 5,583,679 (Ito et al.), U.S. Pat. No. 5,853,801 (Suga et al.), U.S. Pat. No. 5,619,352 (Koch et al.), U.S. Pat. No. 5,978,055 (Van De Witte et al.), and U.S. Pat. No. 6,160,597 (Schadt et al.). A compensator based on discotic liquid crystals according to U.S. Pat. No. 5,583,679 (Ito et al.) and U.S. Pat. No. 5,853,801 (Suga et al.), which discotic liquid crystals have negative birefringence, is widely used. Although it offers improved contrast over wider viewing angles, it suffers larger color shift for gray level images compared to a compensator made of liquid crystalline materials with positive birefringence, according to Satoh et al. (xe2x80x9cComparison of nematic hybrid and discotic hybrid films as viewing angle compensator for NW-TN-LCDsxe2x80x9d, SID 2000 Digest, pp. 347-349, 2000).
In order to achieve improved performance in the contrast ratio while limiting color shift, one alternative is to use a pair of crossed liquid crystal polymer films (LCP) on each side of a liquid crystal cell, as discussed by Chen et al. (xe2x80x9cWide Viewing Angle Photoaligned Plastic Filmsxe2x80x9d, SID 99 Digest, pp.98-101 1999). A liquid pre-polymer is also used to make a layer for aligning or orientating the LCP layer. This paper states that xe2x80x9csince the second liquid pre-polymer/LCP retarder film is coated directly on top of the first LCP retarder film, the total thickness of the final wide-view retarder stack is only a few microns thin.xe2x80x9d Although this method provides a very compact optical compensator, one of the challenges of this method is to make two LCP layers crossed, particularly in a continuous roll-to-roll manufacturing process.
U.S. Pat. No. 5,853,801 (Suga et al.) teaches a continuous process of preparing an optical compensator by, for example, coating a transparent resin layer on a transparent support, subjecting the layer to a brushing treatment to form an orientation layer, coating a solution of a discotic liquid-crystal compound upon the orientation layer, drying the solution of the liquid-crystal compound to form a coated layer, and heating the coated layer to form a discotic nematic phase.
In EP 646829 A1, the optical compensator is prepared by a process similar to that described by Suga et al. but wherein the steps are performed discontinuously. Thus, the process is not suitable for high volume industrial production.
U.S. Pat. No. 6,160,597 (Schadt et al.) discloses steps for making an optical compensator using discontinous or independent steps comprising drying, heating, and cooling treatments for periods as long as one hour or more, and therefore this process is also not suitable for high-volume industrial production.
A problem in the manufacture of optical compensators for widening the viewing-angle characteristics of liquid crystal displays has been preventing defects, especially in a process suitable for industrial production.
In particular, in the preparation of optical compensators in which a series of aligning layers and anisotropic liquid crystal layers have been coated on a support, and in which an anisotropic liquid crystal (LC) layer is UV cured before the application of the next aligning layer, achieving a sufficient and consistent contrast ratio has been difficult. Furthermore, poor adhesion has been encountered between the aligning layer and the LC layer.
The invention provides an improved process for making an optical compensator for use with liquid crystal displays, which process involves forming an orientation layer and an anisotropic nematic liquid crystal (LC) layer on a support made from cellulose triacetate, glass, or other suitable material. Each aligning layer is photo-aligned to become the orientation surface (command surface) before the application of the next anisotropic layer. In particular, it has been found that heat treating the photo-aligned orientation layer after it has been dried improves the achievable tilt angle of the orientation layer. Furthermore, such a post-dry heat treatment has also been found to improve the adhesion.
The present process can be used to make an optical compensator film that widens the viewing angle characteristics of liquid crystal displays, and in particular of Twisted Nematic (TN), Super Twisted Nematic (STN), Optically Compensated Bend (OCB), In Plane Switching (IPS), or Vertically Aligned (VA) liquid crystal displays.
There is provided by the invention a process for the preparation of a continuous optical compensator sheet comprising the continuous steps of:
i) coating, in a solvent, a liquid of a photo-alignable resin for forming an orientation layer onto a surface of a continuous transparent film;
ii) drying the resin-containing coating to form a transparent resin layer;
iii) subjecting the transparent resin layer to a photo-alignment treatment whereby an orientation layer is formed;
iv) heat treating the orientation layer wherein the heat treatment can occur either before or after, or both before and after, the alignment process;
v) coating a liquid-crystalline compound in a solvent on the orientation layer to form a liquid-crystal-containing coating;
vi) drying the liquid-crystal-containing coating to form a liquid-crystal-containing layer;
vii) thermally treating the liquid-crystal-containing layer in order to form an anisotropic liquid crystal layer exhibiting its functional phase, preferably the nematic phase of a nematic liquid-crystal compound.
An preferred optional further step comprises winding up the transparent film having the orientation layer and the anisotropic liquid crystal layer. Additional preferred embodiments of the above-mentioned process for preparing an optical compensator film are as follows:
1) The above process wherein step i) is conducted by using a slot-hopper method to apply the resin-containing coating onto the transparent support. In other embodiments, a slide, gravure, rod, or roll coating method can be is used to coat the resin material.
2) The above process wherein step iii) is conducted by exposing a surface of the transparent resin layer of the continuous transparent film to polarized ultraviolet light to impart orientation to the transparent resin layer.
3) The above process wherein step v) is conducted by coating a polymerizable or crosslinkable nematic compound having liquid crystalline properties, in a solvent, onto the orientation layer; and step vii) comprises cooling the coated layer to form a nematic phase and thereafter exposing the layer to light so as to cure the nematic compound.
4) The above process wherein step ii) is conducted by vaporizing the solvent from the coating by applying heated gas or infrared radiation and/or microwave radiation to both sides of the transparent film, or bringing the side opposite of the transparent film opposite the coating into contact with at least one heated roller, preferably a plurality of heated rollers.
5) The above process wherein step ii) is conducted by vaporizing the solvent from the coating by applying heated gas or infrared radiation or microwave radiation to only the side of the transparent film opposite the coated layer.
In another embodiment, there is provided in accordance with the present invention a process for the preparation of a continuous optical compensator sheet comprising at least two orientation layers, which process comprises the continuous steps of:
i) coating, in a solvent, a liquid of a photo-alignable resin for forming a second orientation layer onto a surface of a continuous transparent film already having a first orientation layer and an anisotropic liquid-crystalline layer thereon (i.e., a continuous transparent film having the orientation layer and the anisotropic liquid-crystal layer thereon);
ii) drying the coating to form a transparent resin layer; and
iii) subjecting the transparent resin layer to a photo-alignment treatment, whereby a second orientation layer is formed;
iv) heat treating the second orientation layer wherein the heat treatment can occur either before or after, or both before or after, the alignment step iii);
v) coating an anisotropic liquid-crystalline compound, in a solvent, onto the second orientation layer to form a liquid-crystal-containing layer;
vi) drying the a liquid-crystal-containing layer;
vii) thermally treating the a liquid-crystal-containing layer in order to form an anisotropic liquid-crystal-containing layer having a functionally oriented phase;
viii) optionally winding up the compensator sheet comprising the transparent support, the orientation layers, and the anisotropic liquid-crystal layers.
Preferred embodiments of the above-mentioned process for preparing an optical compensator sheet are as follows:
1) The above process wherein step i) is conducted by continuously bringing the transparent support having the orientation layer and the anisotropic liquid crystal layer into coating contact with a slot hopper to coat the resin-containing material. In other embodiments, a slide gravure, rod, or roll coating method is used to coat the liquid.
2) The above process wherein step iii) is conducted by exposing a surface of the transparent resin layer to polarized ultraviolet light to impart orientation to the transparent resin layer, whereby a second orientation layer is formed. Preferably, the orientation of the second orientation layer is such that the optical axis of the first anisotropic liquid crystal layer is positioned orthogonally relative to the respective optical axis of the first anisotropic layer (to which the second orientation layer was applied thereon about an axis perpendicular to the plane of the support.
3) The above process wherein the step v) comprises coating a polymerizable or crosslinkable nematic compound having liquid crystalline properties, in a solvent, onto the second orientation layer to form a second anisotropic layer; and step vii) comprises cooling the liquid-crystal containing layer to form a nematic phase, and thereafter exposing the nematic phase to light so as to cure the nematic compound.
4) The above process wherein step ii) is conducted by vaporizing the solvent from the coating by applying heated gas or infrared or microwave radiation to both sides of the transparent film, or bringing the side of the transparent film opposite the coating into contact with one or more heated rollers.
5) The above process wherein step ii) is conducted by vaporizing the solvent from the coated layer by applying heated gas or infrared rays or microwave rays to only the side of the transparent film opposite the coating.
6) The above process wherein the step vii) is conducted by cooling the liquid-crystal-containing layer to form a positive birefringence nematic phase, whereby an anisotropic nematic liquid-crystal-containing layer is formed.